1. Field of the Invention
The present invention relates to a method for fabricating a liquid crystal display device, and more particularly, to a method for fabricating a color filter array substrate of a liquid crystal display device.
2. Discussion of the Related Art
With the development of an information society, various types of display devices have been in great demand. Accordingly, much effort has been made to research and develop various flat display devices, such as liquid crystal displays (LCDs), plasma display panels (PDPs), electroluminescent displays (ELDs), and vacuum fluorescent displays (VFDs). Some species of the flat display devices have been already utilized as displays of various types of equipment. Among those, the LCD device has been the most popular due to thin profile, lightweight, low power consumption, etc., and has been regarded as a substitute for the cathode ray tube (CRT). For example, the LCD device may be utilized as a display for a lap-top computer, a pocket computer, or any monitor of a display device mountable on an automobile or a television screen to display broadcasting signals.
The LCD device mainly includes a color filter substrate as an upper substrate, a thin film transistor (TFT) array substrate as a lower substrate, which face each other, and a liquid crystal layer having a dielectric anisotropy formed between the two substrates. The LCD device is operated by applying a voltage to a corresponding unit using a switching operation of a TFT added to several hundred thousand pixels through a pixel selecting address line.
The color filter substrate includes red, green, and blue color filter layers sequentially aligned and each representing a corresponding color, a black matrix separating each of the red, green, and blue color filter layers and blocking light therebetween, and a common electrode for applying a voltage to the liquid crystal layer. Each of the three color filter layers is independently operated, and the color of a single pixel is represented by any one of the three colors or by a combination of at least two of the three colors.
Related art methods for fabricating the color filter array substrate include a dye method and a pigment method depending upon the material of an organic filter utilized in a process for fabricating the color filter layer. The fabricating process may include a coloring (or dyeing) method, a dispersion method, a coating method, an electrodeposition method, and an ink-jet method. Recently, the pigment dispersion method has been widely utilized for fabricating the color filter layer, and a description thereof will now be given in detail below.
FIGS. 1A to 1G illustrate cross-sectional views showing the related art pigment dispersion method. As shown in FIG. 1A, an insulating substrate 11 is washed, and then either a metal, such as chromium oxide (CrOx) or chromium (Cr), having an optical density of at least 3.5, or a carbon group organic material is deposited on the insulating substrate 11. After that, the insulating substrate 1 is patterned through a photolithography process to form a black matrix 13. Herein, the black matrix 13 is formed so that the edge portion of a unit pixel and the region for forming a TFT correspond to each other, thereby blocking the light emitted from a light source in a region having an unstable electric field.
After the black matrix 13 is thus formed, a color resist including a pigment for representing colors is deposited with a thickness in the range of about 1 to 3 micrometers (μm) on the entire surface of the insulating substrate 11. A first color resist layer 14a dyed in red is first deposited on the insulating substrate 11 to completely cover the black matrix 13.
The color resist can be deposited using either of a spinning method or a roll coating method. The spinning method drops an adequate amount of the color resist on the insulating substrate 11, and then the insulating substrate 11 is spun at a high speed to uniformly disperse the color resist onto its entire surface. On the other hand, the roll coating method spreads the color resist onto a roll to transfer/print it onto the insulating substrate 11. The essential elements of the color resist include a photopolymerization initiator, which is a photosensitive composition such as a photoresist, a monomer, a binder, and an organic pigment.
Subsequently, as shown in FIG. 1B, apart from a specific region of the first color resist layer 14a (i.e., a region remaining for a pattern), an opaque portion of a mask 17 masks the entire surface of the first color resist layer 14a, and then UV light rays are irradiated thereon to partially expose the first color resist layer 14a. Generally, the color resist for color filters has a negative characteristic of removing a non-exposed portion of the color resist. Therefore, the specific region for forming the pattern of the color resist is exposed through a transparent portion of the mask 17. The light exposing process includes a proximity process exposing a main substrate to sunlight, a stepper process repeatedly exposing a reduced pattern, and a mirror reflection process exposing the substrate 11 by reflecting the mask pattern. A simple matrix LCD prioritizing productivity uses the proximity process, which has poor accuracy but a fast treatment speed. On the other hand, the active matrix LCD requiring high accuracy uses either the stepper process or the mirror reflection process.
As shown in FIG. 1C, the first color resist layer 14a having its photochemical structure modified by exposure is dipped into a developing solution and patterned. The first color resist layer 14a is thus developed to form a first color filter layer 15b as shown in FIG. 1C. The first color filter layer 15a is dyed in red and the non-exposed portion thereof is removed. Thereafter, the first color filter layer 15a is hardened at a high temperature condition of about 230° C. The developing process is performed using one of a dipping method, a bubble method, and a shower spray method.
However, a step difference is generated at an overlapping portion of the first color filter layer 15a and the black matrix 13, and such a problem can be worsened if the black matrix 13 is formed of a thick organic layer of a carbon group. Therefore, to planarize the first color filter layer 15a, a process of depositing an overcoat layer having excellent planarization characteristics is required in a later process.
As shown in FIG. 1D, a second color resist layer 14b dyed in green is deposited on the entire surface of the insulating substrate 11 including the red first color filter layer 15a. Then, apart from a specific region of the second color resist layer 14b, an opaque portion of the mask 17 (which is physically the same mask as that used for patterning the first color resist 14a, but is shifted in position) masks the entire surface of the second color resist layer 14b, and then UV light rays are irradiated thereon to partially expose the second color resist layer 14b. Thereafter, the second color resist layer 14b having its photochemical structure modified by exposure is developed to form a second color filter layer 15b dyed in green, as shown in FIG. 1E. The second color filter layer 15b is formed in a pixel adjacent to the first color filter layer 15a, wherein the black matrix 13 is placed therebetween.
Then, a third color resist layer 14c dyed in blue is deposited on the entire surface of the insulating substrate 11 including the first and second color filter layers 15a and 15b. Then, apart from a specific region of the third color resist layer 14c, an opaque portion of the mask 17 masks the entire surface of the color resist layer 14c, and then UV light rays are irradiated thereon to partially expose the third color resist layer 14c. Thereafter, the third color resist layer 14c having its photochemical structure modified by exposure is developed to form a third color filter layer 15c dyed in blue, as shown in FIG. 1F.
The third color filter layer 15c is formed in a pixel adjacent to the second color filter layer 15b, wherein the black matrix 13 is placed therebetween, thereby completing a color filter layer 15 having a multi-layered structure of red (R), green (G), and blue (B) color filter layers 15a, 15b and 15c. Generally, the color filter layer 15 is formed in the order of R, G, and B.
Subsequently, as shown in FIG. 1G, to protect and planarize the color filter layer 15, an acrylic resin or a polyamide resin is utilized to deposit a planarization layer on the entire surface of the insulating substrate 11 including the color filter layer 15 by means of a spin-coating method, thereby forming a transparent overcoat layer 16.
Finally, an indium tin oxide (ITO) layer 18, which is a transparent material having excellent transmissivity, conductivity, and chemical and thermal stability, is deposited on the overcoat layer 16 by means of a sputtering method as a common electrode. The common electrode 18 operates the liquid crystal layer as well as the pixel electrode formed on the TFT array substrate. Herein, depending upon the type of the LCD device, the process of forming the ITO layer 18 on the overcoat layer 16 may be omitted. Thus, the method for fabricating the color filter array substrate including the black matrix 13, the color filter layer 15, and the overcoat layer 16 or the common electrode 18 is completed.
However, the related art method for fabricating the color filter array substrate has the following disadvantages. As each of the red (R), green (G), and blue (B) color filter layers is formed through a separate process, a step difference may be formed between the R, G, and B color filter layers depending upon the characteristic of the color filter and the degree of exposure to light. As a result, to planarize the color filter layer having a step difference, a process of selecting a transparent organic material having an excellent planarization characteristic and forming an overcoat layer is required, thereby complicating the fabrication process and increasing the fabrication costs and time.
Additionally, by forming the overcoat layer, the color filter array substrate becomes thicker, thereby resulting in limitations for forming thin and lightweight liquid crystal panels. Also, since a passivation layer cannot be rubbed at an area having the step difference formed in the color filter layer, the alignment of liquid crystal molecules in the corresponding area becomes difficult, thereby causing the problem of disclination.